420) THE DEVELOPMENT OF THE CHICK 
notochord runs continuously through the centra of the four 
vertebree shown. It is constricted intravertebrally and expanded 
intervertebrally, so that the vertebral bodies are amphiccelous. 
The intervertebral discs are not shown. Each vertebral centrum 
consists of united cephalic and caudal parts; the arches arising 
from the centra are likewise double, but the caudal component 
diminishes posteriorly and is incomplete in the last vertebra 
shown. <A pre- and postzygapophysis is formed on each arch. 
As it is possible to follow the sclerotomal components of the 
primitive vertebre up to this stage continuously, there can be 
no reasonable doubt that they correspond to the divisions shown 
oie 
by the staining in the cartilaginous vertebre of Fig. 237. The 
ceph vA. 
CaUL VA 
Fic. 237.— The right side of four bisected vertebrie of the trunk 
of an 8-day chick. (After Schauinsland.) 
eaud. v. A., Caudal division of vertebral arch. ceph. v. A., 
Cephalie division of vertebral arch, N’ch., Notochord. 
successive vertebra have persistent membranous connections in 
the regions of the neural spines, zygapophyses and centra. These 
are shown in Figs. 238 and 239 (cf. also Fig. 150); they are con- 
tinuous with the perichondrium and all are derived from unchon- 
drified parts of the original membranous vertebre. 
Atlas and Axis (epistropheus). The first and second verte- 
bre agree with the others in the membranous stage. But, when 
chondrification sets in, the hypochordal bar of the first vertebra does 
not fuse with the body, but remains separate and forms its floor 
(Figs. 238 and 239). The body of the first vertebra chondrifies 
separately and is attached by membrane to the anterior end of 
the body of the second vertebra, representing in fact the odon- 
toid process of the latter. It has later a separate center of ossi- 
fication, but fuses subsequently with the body of the second 
vertebra, forming the odondoid process (Fig. 240). 
